Hi Yafang, On Mon, Aug 12, 2024 at 10:29:25AM GMT, Yafang Shao wrote: > We want to eliminate the use of __get_task_comm() for the following > reasons: > > - The task_lock() is unnecessary > Quoted from Linus [0]: > : Since user space can randomly change their names anyway, using locking > : was always wrong for readers (for writers it probably does make sense > : to have some lock - although practically speaking nobody cares there > : either, but at least for a writer some kind of race could have > : long-term mixed results > > - The BUILD_BUG_ON() doesn't add any value > The only requirement is to ensure that the destination buffer is a valid > array. > > - Zeroing is not necessary in current use cases > To avoid confusion, we should remove it. Moreover, not zeroing could > potentially make it easier to uncover bugs. If the caller needs a > zero-padded task name, it should be explicitly handled at the call site. > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Link: https://lore.kernel.org/all/CAHk-=wivfrF0_zvf+oj6==Sh=-npJooP8chLPEfaFV0oNYTTBA@xxxxxxxxxxxxxx [0] > Link: https://lore.kernel.org/all/CAHk-=whWtUC-AjmGJveAETKOMeMFSTwKwu99v7+b6AyHMmaDFA@xxxxxxxxxxxxxx/ > Suggested-by: Alejandro Colomar <alx@xxxxxxxxxx> > Link: https://lore.kernel.org/all/2jxak5v6dfxlpbxhpm3ey7oup4g2lnr3ueurfbosf5wdo65dk4@srb3hsk72zwq > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx> > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> > Cc: Christian Brauner <brauner@xxxxxxxxxx> > Cc: Jan Kara <jack@xxxxxxx> > Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> > Cc: Matus Jokay <matus.jokay@xxxxxxxx> > Cc: Alejandro Colomar <alx@xxxxxxxxxx> > Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx> > --- > fs/exec.c | 10 ---------- > fs/proc/array.c | 2 +- > include/linux/sched.h | 31 +++++++++++++++++++++++++------ > kernel/kthread.c | 2 +- > 4 files changed, 27 insertions(+), 18 deletions(-) > > diff --git a/fs/exec.c b/fs/exec.c > index a47d0e4c54f6..2e468ddd203a 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1264,16 +1264,6 @@ static int unshare_sighand(struct task_struct *me) > return 0; > } > > -char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk) > -{ > - task_lock(tsk); > - /* Always NUL terminated and zero-padded */ > - strscpy_pad(buf, tsk->comm, buf_size); This comment is correct (see other comments below). (Except that pedantically, I'd write it as NUL-terminated with a hyphen, just like zero-padded.) > - task_unlock(tsk); > - return buf; > -} > -EXPORT_SYMBOL_GPL(__get_task_comm); > - > /* > * These functions flushes out all traces of the currently running executable > * so that a new one can be started > diff --git a/fs/proc/array.c b/fs/proc/array.c > index 34a47fb0c57f..55ed3510d2bb 100644 > --- a/fs/proc/array.c > +++ b/fs/proc/array.c > @@ -109,7 +109,7 @@ void proc_task_name(struct seq_file *m, struct task_struct *p, bool escape) > else if (p->flags & PF_KTHREAD) > get_kthread_comm(tcomm, sizeof(tcomm), p); > else > - __get_task_comm(tcomm, sizeof(tcomm), p); > + get_task_comm(tcomm, p); LGTM. (This would have been good even if not removing the helper.) > > if (escape) > seq_escape_str(m, tcomm, ESCAPE_SPACE | ESCAPE_SPECIAL, "\n\\"); > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 33dd8d9d2b85..e0e26edbda61 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1096,9 +1096,11 @@ struct task_struct { > /* > * executable name, excluding path. > * > - * - normally initialized setup_new_exec() > - * - access it with [gs]et_task_comm() > - * - lock it with task_lock() > + * - normally initialized begin_new_exec() > + * - set it with set_task_comm() > + * - strscpy_pad() to ensure it is always NUL-terminated The comment above is inmprecise. It should say either "strscpy() to ensure it is always NUL-terminated", or "strscpy_pad() to ensure it is NUL-terminated and zero-padded". > + * - task_lock() to ensure the operation is atomic and the name is > + * fully updated. > */ > char comm[TASK_COMM_LEN]; > > @@ -1912,10 +1914,27 @@ static inline void set_task_comm(struct task_struct *tsk, const char *from) > __set_task_comm(tsk, from, false); > } > > -extern char *__get_task_comm(char *to, size_t len, struct task_struct *tsk); > +/* > + * - Why not use task_lock()? > + * User space can randomly change their names anyway, so locking for readers > + * doesn't make sense. For writers, locking is probably necessary, as a race > + * condition could lead to long-term mixed results. > + * The strscpy_pad() in __set_task_comm() can ensure that the task comm is > + * always NUL-terminated. This comment has the same imprecission that I noted above. > Therefore the race condition between reader and > + * writer is not an issue. > + * > + * - Why not use strscpy_pad()? > + * While strscpy_pad() prevents writing garbage past the NUL terminator, which > + * is useful when using the task name as a key in a hash map, most use cases > + * don't require this. Zero-padding might confuse users if it’s unnecessary, > + * and not zeroing might even make it easier to expose bugs. If you need a > + * zero-padded task name, please handle that explicitly at the call site. > + * > + * - ARRAY_SIZE() can help ensure that @buf is indeed an array. > + */ > #define get_task_comm(buf, tsk) ({ \ > - BUILD_BUG_ON(sizeof(buf) != TASK_COMM_LEN); \ > - __get_task_comm(buf, sizeof(buf), tsk); \ > + strscpy(buf, (tsk)->comm, ARRAY_SIZE(buf)); \ > + buf; \ > }) > > #ifdef CONFIG_SMP > diff --git a/kernel/kthread.c b/kernel/kthread.c > index f7be976ff88a..7d001d033cf9 100644 > --- a/kernel/kthread.c > +++ b/kernel/kthread.c > @@ -101,7 +101,7 @@ void get_kthread_comm(char *buf, size_t buf_size, struct task_struct *tsk) > struct kthread *kthread = to_kthread(tsk); > > if (!kthread || !kthread->full_name) { > - __get_task_comm(buf, buf_size, tsk); > + strscpy(buf, tsk->comm, buf_size); > return; > } Other than that, LGTM. -- <https://www.alejandro-colomar.es/>
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